{{Short description|Data structure in computer science}} {{about|the data type|the mathematical concept|Multivalued function|the mapping website|Multimap.com}} {{more citations needed|date=February 2022}} In computer science, a '''multimap''' (sometimes also '''multihash''', '''multidict''' or '''multidictionary''') is a generalization of a map or associative array abstract data type in which more than one value may be associated with and returned for a given key. Both map and multimap are particular cases of containers (for example, see C++ Standard Template Library containers). Often the multimap is implemented as a map with lists or sets as the map values.

==Examples== * In a student enrollment system, where students may be enrolled in multiple classes simultaneously, there might be an association for each enrollment of a student in a course, where the key is the student ID and the value is the course ID. If a student is enrolled in three courses, there will be three associations containing the same key. * The index of a book may report any number of references for a given index term, and thus may be coded as a multimap from index terms to any number of reference locations or pages. * Querystrings may have multiple values associated with a single field. This is commonly generated when a web form allows multiple check boxes or selections to be chosen in response to a single form element.

==Language support==

===C++=== C++'s Standard Template Library provides the <code>multimap</code> container for the sorted multimap using a self-balancing binary search tree,<ref>{{cite web | url = http://www.sgi.com/tech/stl/Multimap.html | title = multimap<Key, Data, Compare, Alloc> | work = Standard Template Library Programmer's Guide | publisher = Silicon Graphics International}}</ref> and SGI's STL extension provides the <code>hash_multimap</code> container, which implements a multimap using a hash table.<ref>{{cite web | url = http://www.sgi.com/tech/stl/hash_multimap.html | title = hash_multimap<Key, HashFcn, EqualKey, Alloc> | work = Standard Template Library Programmer's Guide | publisher = Silicon Graphics International}}</ref>

As of C++11, the Standard Template Library provides the <code>unordered_multimap</code> for the unordered multimap.<ref>{{cite web |url= http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf |title = Working Draft, Standard for Programming Language C++ | page= 7807}}</ref>

===Dart=== Quiver provides a Multimap for Dart.<ref>{{cite web | url = https://pub.dev/documentation/quiver/latest/quiver.collection/Multimap-class.html | title = Multimap | work = Quiver API docs}}</ref>

===Java=== Apache Commons Collections provides a MultiMap interface for Java.<ref>{{cite web | url = https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/MultiMap.html | title = Interface MultiMap | work = Commons Collections 3.2.2 API, Apache Commons}}</ref> It also provides a MultiValueMap implementing class that makes a MultiMap out of a Map object and a type of Collection.<ref>{{cite web | url = https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.2/org/apache/commons/collections/map/MultiValueMap.html | title = Class MultiValueMap | work = Commons Collections 3.2.2 API, Apache Commons}}</ref>

Google Guava provides a Multimap interface and implementations of it.<ref>{{cite web | url = http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html | title = Interface Multimap<K,V> | work = Guava Library 2.0 | access-date = 2013-01-01 | archive-url = https://web.archive.org/web/20130115105942/http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html | archive-date = 2013-01-15 | url-status = dead }}</ref>

===Kotlin=== Kotlin does not have explicit support for multimaps,<ref>{{cite web | url = https://www.baeldung.com/kotlin/multimap| title = Implement a MultiMap in Kotlin &#124; Baeldung on Kotlin| date = 5 December 2023}}</ref> but can implement them using Maps with containers<ref>{{cite web | url = https://developer.android.com/training/data-storage/room/accessing-data#multimap| title = Accessing data using Room DAOs}}</ref> for the value type. E.g. a <code>Map<User, List<Book>></code> can associate each User with a list of Books.

===Python=== Python provides a <code>collections.defaultdict</code> class that can be used to create a multimap. The user can instantiate the class as <code>collections.defaultdict(list)</code>.

===OCaml=== OCaml's standard library module <code>Hashtbl</code> implements a hash table where it's possible to store multiple values for a key.

===Scala=== The Scala programming language's API also provides Multimap and implementations.<ref>{{cite web | url = http://www.scala-lang.org/api/current/scala/collection/mutable/MultiMap.html | title = Scala.collection.mutable.MultiMap | work = Scala stable API}}</ref>

==See also== * Multiset for the case where same item can appear several times

==References== <references/>

{{Data structures}}

Category:Associative arrays Category:Abstract data types